1. Field of the Invention
The invention relates to the field of connectors and more particularly to connectors between mother and daughter boards.
2. Brief Description of the Prior Art
Ground planes have been included in flexible printed circuits heretofore: those having one plane being known as microstrip flexible printed circuits, and those having two planes sandwiching the logic traces being known as stripline flexible printed circuits. Typically, the ground plane connections to other circuits or boards include through-hole type connections. This avoids the lead alignment problem discussed below.
While ground planes have been included in connectors between mother and daughter boards, see, e.g., Franck U.S. Pat. No. 3,582,865 and ground plane 39 at FIG. 4, prior flex circuit connectors between mother and daughter boards which include MIL-C-28754 Type III head connectors have not included ground planes. In FIG. 1, the head 10 is the prior art mil-spec head, and a prior flex connector looked like the flex connector 20 of the preferred embodiment without the ground plane. The flex connector of the prior art includes holes in a base portion which mate with pins 14 inside bushings 12 on the head 10. After mating, the pins are soldered. The flex connector of the prior art is also soldered to the printed circuit daughter board 40 via leads 30 which are soldered to pads, not shown, mounted on the printed circuit board 40, after which lead support 54 is cut off.
Applicant believes one reason ground planes have not heretofore been included in flex circuit connectors is that the ground plane stiffens the flex connector, which may lead to increased stress-related lead fractures under the vibrations of military conditions.
A problem faced by those who wish to include a ground plane in flex circuit connectors is the difficulty in aligning the leads from the logic traces and ground plane, separated as they must be by an intervening insulating layer. FIG. 2 shows the separation of the ground plane 60 from logic traces 70 by an insulating layer 80. FIG. 3 shows a plan view of the logic traces 70 and the leads 72 connected thereto. Interspersed among the logic trace leads are ground leads 74. In the illustrated example, ground leads coming from the reverse side of the insulating layer 80 are slightly offset from the leads of the logic traces.
Despite these difficulties, ground planes in flex circuit connectors are increasingly required because the speed of operations of the circuits are increasing, leading to circuit impedance mismatches between mother and daughter boards and further causing localized ground potential shift due to the switching of numerous circuits at one time. Impedance mismatches and ground potential shifts may cause marginal signals to be misinterpreted.